It is recognized in the art of degreasing that the mixing of solvent vapor with air is very costly in terms of solvent loss. Solvent/air mixtures of the kind with which the present invention deals are to be distinguished from vapors which are essentially wholly solvent and appreciably more dense and behave differently. Because halogenated hydrocarbons which are most frequently employed as solvent in degreasing systems are heavier than air, trichloroethylene being an example, the vapor can be controlled by a simple condenser coil or jacket near the top of the degreasing tank. However, when small concentrations of solvent vapor and air intermix, the much lighter combined mixture will be carried off by normal air movement. Even in a quiet atmosphere, the loss due to diffusion in air is considerable. Drafts or improper introduction and removal of the work pieces aggravate the vapor (solvent) loss substantially at this rate.
The normal diffusion of solvent in air is nearest the theoretical minimum when the degreaser is in an area where the working atmosphere is as quiet as possible. Installation of baffles or shields helps to control air movement. Even under ideal conditions, a degreaser is constructed with a freeboard (height of sidewall above the vapor line) preferably of the order of 60% or more of the machine width.
In operation, the degreaser should be large enough and have enough heat input to handle the normal work load. Overloading increases solvent loss. For example, as the work basket is inserted into the degreaser opening, there is considerable intermixing of air and vapor and the resulting turbulence increases the tendency for vapor loss. To stem this very considerable potential loss of solvent by diffusion, a second condenser near the top of the degreaser, which is sometimes referred to as a freeboard chiller, has been devised to suppress the tendency of solvent vapors to escape through the open top of the degreasing apparatus. A vapor condenser or freeboard chiller system of this kind is disclosed in the Rand U.S. Pat. No. 3,373,177 which utilizes a second condenser means above a first condenser and below the upper edge of the open top degreasing vessel into which the parts to be treated are lowered and raised when the parts are withdrawn.
In a degreaser system, vapors are generated in an open topped vessel by boiling a solvent in a heated chamber. The generated vapors rise within the vessel and contact the work piece(s) to be cleansed, generally, metal parts, supported upon a work rest within the vessel. The vapors will dissolve the grease on the metal parts.
The vessel used in degreasing apparatus of this kind is open to provide ready access to the interior of the vessel. The use of an open access vessel in degreasing apparatus, while of great convenience from the standpoint of practical access, has caused several problems. These include: (1) excessive loss of the expensive solvent which is dispersed with air and lost rendering the degreasing operation costly; (2) noxious solvent odors emitted from the apparatus; and (3) a toxic hazard to personnel through air pollution of the work place and the environment.
As noted in U.S. Pat. No. 3,375,177, the objective of the freeboard chiller is to control the vapor/air mixtures generated by the apparatus, but not condensed by the condensing coils or water jacket, before they are expelled into the atmosphere. Low temperature (less than 0.degree. C.) refrigeration coils have been installed in the freeboard zone of degreasers above the primary condensing coils. The cold air blanket produced by these coils acts as a thermal inversion tending to trap rising air/solvent vapor mixtures and effectively condensing a portion of the solvent vapor, thus preventing its escape from the degreasers. Substantial loss reductions have been reported with normal loss reductions of 40% being represented in the industry.
The known low temperature freeboard chillers, such as the kind described in U.S. Pat. No. 3,375,177 are currently incorporated in new degreasers at the time of manufacture, and although such freeboard chillers may be retrofitted on existing degreasers, it is necessary that trained factory personnel be employed for installation. During installation, the trained factory person must cut, fit, mount and solder finned refrigeration tubing around the inner periphery of the degreaser in situ in the freeboard zone and then connect said tubing to a low temperature refrigeration condensing unit. The system must then be checked for leaks and charged with a refrigerant gas. Such installation requires sizeable expenditures. Downtime is also a significant drawback. Also, because of the exacting nature of this type of installation, relative to the fitting of components and preclusion of leaks, as well as the necessity to operationally check the equipment, the purchase of the necessary components for self-assembly by the degreaser owner, as a rule, has not been attempted. It is evident that the installed cost of freeboard chillers is considerably higher due to travel and living costs for the factory trained personnel which must of necessity be borne by the purchaser.
Another limitation in the matter of installation of current freeboard chillers is the reduction in the top opening of the degreaser which results and which can preclude the retrofitting of the solvent saving device due to insufficient clearance for existing workload sizes or the reduction in the maximum workload size that can be placed in a degreaser.
It is obvious from the above that it would be advantageous to develop a low temperature compact freeboard chiller that could be shipped as a completed, sealed with refrigerant included and pretested unit, i.e., a module, that could be installed or retrofitted by the ultimate user, thus eliminating the very substantial travel and living costs from the purchase price required when factory trained personnel must install such systems.